Semiconductor lasers generally used are of a Fabry-Perot (FP) type that includes an optical resonator sandwiched between mirrors formed at both end faces of an active layer. However, the FP laser oscillates at a wavelength that satisfies the standing-wave condition, and thereby tends to operate in a multi-longitudinal mode. In particular, as current or temperature changes, the lasing wavelength of the laser varies, which results in a change in the optical intensity.
Meanwhile, for the purpose of optical communications, gas sensing, and the like, lasers are required to exhibit high wavelength stability and to oscillate in a single mode. For this reason, a distributed feedback (DFB) laser and a distributed Bragg reflector (DBR) laser have been developed. These lasers are designed to include a diffraction grating formed in a semiconductor and to oscillate at a specific wavelength using the wavelength dependency of the grating.
Examples of semiconductor lasers that achieve adequate wavelength stability can include a DBR laser and a DFB laser, which have a grating monolithically formed in a semiconductor laser, and an external resonator laser having a fiber grating (FBG) grating attached to the outside of the laser. These are based on the principle that part of the laser light is returned to the lasers by mirrors with the wavelength selectivity using Bragg reflection to achieve a stable wavelength operation.
The DBR laser achieves a resonator by forming convex and concave portions in a waveguide surface located on an extended line of a waveguide in an active layer to thereby fabricate mirrors in conformity with Bragg reflection (see Patent Document 1 (Japanese Unexamined Patent Application Publication S49(1974)-128689A); and Patent Document 2 (Japanese Unexamined Patent Application Publication S56(1981)-148880A)). In such a laser, diffraction gratings are provided on both ends of an optical waveguide layer, whereby light emitted from the active layer propagates through the optical waveguide layer, and part of the light is reflected by the diffraction gratings to return to a current injection portion and then to be amplified. The light with only one wavelength is reflected from the diffraction grating in a determined direction, so that the wavelength of the laser light becomes constant.
As a further application of this laser, an external resonator type semiconductor laser has been developed that includes a resonator formed outside a semiconductor by installing a diffraction grating as a component that differs from the semiconductor. This type of laser is a laser having excellent wavelength stability, temperature stability, and controllability. Examples of the external resonator include a fiber Bragg grating (FBG) (Non-Patent Document 1) and a volume hologram grating (VHG) (Non-Patent Document 2). The diffraction grating is configured separately from the semiconductor laser, which has the feature that the reflectance and the length of the resonator can be designed individually. Thus, the diffraction grating is not influenced by increases in temperature due to the heat generated by current injection, so that the wavelength stability can be further improved. Since the change in the refractive index of the semiconductor depending on the temperature is different from that of the diffraction grating, the diffraction grating can be designed together with the length of the resonator, thereby enhancing the temperature stability of the semiconductor laser.
Patent Document 6 (Japanese Unexamined Patent Application Publication No. 2002-134833A) discloses an external resonator type laser that utilizes a grating formed in a quartz glass waveguide. This document provides the laser with adequate frequency stability that can be used in an environment in which the room temperature changes significantly (e.g. to 30° C. or higher), without a temperature controller. Furthermore, it states that a temperature-independent laser is provided which suppresses mode hopping and has no temperature dependency of its lasing frequency.